The invention relates to a method of closing and/or joining a connecting joint or joining seam between two pieces of galvanized sheet metal.
Zinc coatings are used in sheet metal as protection from corrosion and are widely used in the production of motorcar body sheets. The further processing of such zinc-coated sheets poses difficulties, however, because the welding of such sheets is only possible with limitations with most welding methods. Zinc-coating causes a strong development of welding spatter. The zinc vapor disturbs the welding process during arc welding to a high extent because the arc becomes unstable and leads to increased wear and tear of the cathodes and the cathodic evaporation of the zinc at unfavorable locations. Moreover, the use of tungsten inert-gas welding (TIG welding) which is preferably employed in the case of light metals is strongly impaired by the occurrence of said zinc spatter.
A usual procedure is therefore to produce a joint with mechanical methods (e.g. edge-forming) and to fill up any remaining gaps in the joint with glues in order to strengthen the joint. As a result of the known disadvantages of gluing (e.g. the necessity of slip-free fixture of the parts to be glued or the pretreatment of the surfaces, low permanent strength), soldering methods are also used. Due to the problematic properties of zinc, special solders are used such as copper/silicon solders. These solders lie with their processing temperature in the zone of the melting point of zinc or below, so that the zinc does not evaporate. The disadvantages are the high costs for these special solders and the unpleasant effect that these solders diffuse through the varnish layer after the varnishing and the soldering paths become visible as discolorations.
It is the object of the present invention to provide a method of the kind mentioned above which avoids these disadvantages and maintains the advantages of soldering.
This is achieved in accordance with the invention by filling the connecting joint or covering the joining seam with an additional material selected from the group consisting of zinc, a zinc alloy whose melting point is similar to that of zinc, tin, and a tin alloy by continuously introducing the additional material into the connecting joint or onto the joining seam, and heating the additional material to melting temperature by a plasma jet to melt without reaching the melting temperature of the sheet metal.
It has been seen that zinc and zinc-coated materials can be processed by using a plasma jet without any considerable production of zinc spatter. By using zinc as a joining material a material is used which is generically identical to galvanized sheet steel and has the same melting point as zinc coating, so that a discoloration of the varnishing through diffusion is prevented over long periods of time.
The plasma jet is produced by a plasma jet welding set, with the temperature of the plasma being situated above the melting point of zinc but under the melting point of the workpieces to be joined.
The spatter-free processing of zinc with a plasma jet is caused by the special properties of plasma. Plasma is a moving electric conductor which is flowed through by current and thus forms a concentric electromagnetic field about itself which constricts the plasma jet in its diameter. The zinc introduced into the plasma is ionized and is thus also subjected to the constriction effect, which suppresses the formation of zinc spatter.
The plasma jet welding set need not have any special embodiment. The regulation of the optimal energy supply at which the temperature of the plasma is over the melting point of zinc but is below the melting point of the workpieces to be joined is carried out in such a way that different parameters such as power setting of the source of current, advance speed, shape and size of nozzle, type of cold gas and its flow quantity, distance of the cathode from the workpiece and the surface at the cathode tip are adjusted to one another in a suitable way.
In one embodiment of the invention, the workpieces to be joined are already adhered to one another by mechanical pre-treatment, with said joint being strengthened by closing any still existing connecting joints in accordance with the invention.
If the plasma jet is enclosed by an inert-gas envelope and said inert-gas envelope is adjusted to the shape of the connecting joint or joining seam, the zinc can still be applied on the workpiece. Heated metallic workpieces bond rapidly with oxygen and form an oxide layer which prevents the wetting with the molten zinc. By supplying the zinc under an inert gas cover, oxidation is prevented and the quality of the connection is thus improved. In order to obtain a higher strength, it is also possible to use an alloy made of zinc and nickel for example.
The zinc and zinc alloy can be supplied as a wire, powder or granulate.
During the welding of galvanized sheet metal, the zinc coat, and thus the protection from corrosion, is destroyed along the joining seam. With the method of the invention, the advantageous possibility is offered to apply a zinc coat onto the joining seam without any disturbing zinc spatter by means of a plasma jet and to thus reproduce the protective zinc coating again. This is achieved in such a way that a plasma jet welding set which carries out the galvanization of the joining seam is provided downstream of a first welding set which is responsible for the welding process, with both devices being coupled. The energy transmission onto the material is regulated in the plasma jet welding set in such a way that the melting temperature of the zinc is reached, but not that of the steel sheet. The zinc is supplied in wire form, During the welding process of the first device, it is also possible to use other welding additives. An online re-galvanization of welding paths is thus enabled. The production of zinc spatter is thus avoided by the use of the plasma method.
The method may be used in the production of doors in the car industry in which, among other things, two galvanized pieces of sheet metal are joined together in such a way that the joint forms the edge of the door. This is produced by means of special edge-forming in which a piece of sheet metal with an L-shaped cross section and a piece of sheet metal with a straight cross section are placed above one another in such a way that the straight piece of sheet metal rests in the L-shaped bend of the other piece of sheet metal. The shorter leg of the L-shaped sheet metal is then bent and the plane piece of sheet metal comes to lie in a kind of bracket of the now U-shaped other piece of sheet metal. The joint thus produced can now be strengthened with the help of the method in accordance with the invention. Tin is used as additional material and accordingly applied as a foil in the joining zone and molten by using a plasma jet without the occurrence of any disturbing zinc spatter.